In the Deathly Hallows, when Harry, Hermione and Ron go to get the Diadem, they find Draco, Crabbe and Goyle waiting for them. How did they get in the room when Ginny and Tonks where in there? Even if they had followed the trio in, how did they get to the Diadem before them?
They overheard/seen Harry enter the Room of Requirement and followed him.
“So how did you get in here?” he asked, trying to distract them.
“I virtually lived in the Room of Hidden Things all last year,” said Malfoy, his voice brittle. “I know how to get in.”
“We was hiding in the corridor outside,” grunted Goyle. “We can do Diss-lusion Charms now! And then,” his face split into a gormless grin, “you turned up right in front of us and said you was looking for a die-dum! What’s a die-dum?”
Also, they didn't get to the diadem before Harry; they were behind him:
He skidded to a halt and turned around. Crabbe and Goyle were standing behind him, shoulder to shoulder, wands pointing right at Harry. Through the small space between their jeering faces he saw Draco Malfoy.